The present invention relates to a method of using electrodischarge machining apparatus for reducing the thickness of turbine nozzle walls and particularly to a post-cast operation using EDM apparatus to remove material from an interior wall of a cavity extending within and between opposite ends of the nozzle to reduce thermal stresses and improve casting yields.
In gas turbine designs, axially spaced annular arrays of nozzles form the nozzle stages of the turbine and each array is comprised of nozzle segments arranged circumferentially about the axis of the turbine rotor. Each nozzle segment includes one or more vanes and inner and outer band sections preferably integrally formed of a cast material, for example, a nickel-based alloy. Cavities are cast in the nozzle vane(s) between the band sections for flowing a cooling medium to cool the nozzles. It will be appreciated that a temperature differential exists between the thermal medium flowing in the nozzle cavity and the hot gases of combustion flowing along the hot gas path of the turbine. That temperature differential causes thermal stresses in the material of the nozzle. By reducing the wall thickness between the interior surface of the cavity and the exterior surface of the nozzle and particularly adjacent the leading edge of the nozzle, these thermal stresses caused by the temperature differentials can be minimized. Thin-walled investment castings for nozzles are, however, difficult to manufacture. Tolerance control, as well as material soundness are effected by the wall thickness. Generally, the thinner the wall, the more difficult it is to cast the nozzle, resulting in reduced casting yields. Accordingly, there has developed a need for a process for reducing the wall thickness of turbine nozzles, which not only reduces thermal stresses but which also improves casting yields.
In accordance with a preferred embodiment of the present invention, there is provided a method of removing material from the interior wall of the turbine nozzle to reduce the nozzle wall thickness and hence the thermal induced stresses, while improving casting yields. Particularly, a post-cast electrodischarge machining process is used to remove material from the inner wall of the nozzle cavity. In a preferred embodiment, the EDM process employs a thin electrode disposed within the cavity of the nozzle and movable along the interior wall of the cavity to remove material and hence reduce the wall thickness to a desired thickness. By performing this machining operation subsequent to casting the nozzle, casting tolerance control and material soundness are vastly improved, resulting in increased casting yields, while simultaneously the desired reduction in thermal stresses is obtained by the resulting reduced wall thickness.
In another preferred embodiment of the present invention, an EDM process using an electrode in the form of a profile and a plunge cut can also be employed. In this embodiment, a profile having a surface in conformance with a final contour of the interior wall surface of the cavity is applied against the interior wall of the cavity. As the profile is advanced against the interior cavity wall, material is removed such that the surface of the profile and the interior wall surface become substantially complementary. The profile advance continues until the desired wall thickness is obtained.
In both preferred embodiments, a roughened interior wall surface may also be formed. For example, protuberances may be formed on the electrode surface to provide a roughened thin electrode may be used whereby the movement of the roughened electrode along the interior wall of the cavity forms a final interior wall surface with a complementary roughened surface. The profile similarly may have a roughened surface such that the final contour of the interior wall surface of the cavity will be complementarily roughened similarly as the roughened surface of the profile.
In a preferred embodiment according to the present invention, there is provided a method of forming a thin-walled nozzle for a gas turbine wherein the nozzle has a cavity opening between radial inner and outer opposite ends of the nozzle, comprising the steps of casting the nozzle containing the cavity and removing cast material along a post-cast interior wall of the cavity using an electrodischarge machining apparatus including an electrode to form a final interior wall surface, thereby reducing the thickness of the wall between an exterior surface of the nozzle and the final interior wall surface of the cavity.